WO 03/47093 describes a Chireix power amplifier. Chireix power amplifiers address the problem of non-linearity of amplification of radio frequency signals to high power levels. The basic principle is that amplitude variations of an output signal are realized by controlling the relative phase of component signals that are subsequently combined to form the output signal. This makes it possible to use component signals with constant amplitude that can be amplified to the desired power level by non-linear amplification, without affecting the linearity of the amplitude variations. In more advanced Chireix amplifiers, the amplitude of the component signals is adapted to the desired output amplitude when the desired output amplitude is small. This serves to prevent useless power dissipation associated with the realization of a low amplitude output signal from component signals that nearly cancel each other.
Unfortunately, practical Chireix power amplifiers still suffer from non-linearity problems, which may give rise to parasitic or unintended modulation. For example, when QAM modulation is used (combined amplitude and phase modulation) the modulated data may be found to be damaged due to spurious modulation.